The human uterine cavity is approximately triangular in shape and relatively flat, much like an envelope. The cavity is entered via the endocervical canal. The proximal end of the canal, the external cervical os, opens to the vagina while the distal end, the internal cervical os, opens to the uterine cavity. The tip of the triangular-shaped uterine cavity is located at the internal cervical os, while the base is defined by the openings that lead to the fallopian tubes, the tubal ostia. Sounding the uterus, i.e., determining the length from the fundus of the uterine cavity to the external cervical os, is usually a blind procedure. A physician inserts a uterine sound transcervically and advances the sound until it reaches the top, or fundus of the uterine cavity, i.e., the base of the triangle between the tubal ostia. The length from the interior fundus to the external cervical os can be measured directly using graduations stamped on the shaft of the sound. The physician relies upon tactile feedback to determine when the sound has touched the fundus.
Conventional uterine sounds are constructed from a malleable metal material, approximately 3.5 mm in diameter with a working length of roughly 25 cm, and have a flattened handle portion the physician can grasp. The uterine sound necessarily is substantially rigid in the axial direction and somewhat flexible out of plane, transverse to its axis, in order to reach the fundus and provide the physician the tactile sensation of touching the fundus.
Conventional uterine sounds provide a direct measurement of the sounding length, which includes both the uterine cavity length and the endocervical canal length. However, a physician may need to know the uterine cavity length in order to perform certain medical procedures. Conventional uterine sounds do not provide direct measurement of the uterine cavity length without additional measurements or other calculations.